1. Field of the Invention
This invention relates to photodetector circuits. More specifically, this invention relates to an improvement in photodetector circuits that provides a low noise photodetector with a nominal response time.
While the present invention is described herein with reference to particular embodiments, it is to be understood that the it is not limited thereto. Modifications to the present invention may be made by those having ordinary skills in the art and access to the teachings of this invention without departing from the true spirit and scope thereof.
2. Description of the Prior Art
Many electrical circuits require a load network to facilitate the delivery of an electrical signal. The load network may be any combination of resistors, capacitors, inductors, or other electrical components. The impedance of the network may be chosen to optimize the performance of the circuit.
Invariably, the load network operates to reduce the signal-to-noise ratio (S/N) of the circuit. This deleterious effect can be minimized by judicious design of the load network with careful attention to its input impedance. This presents a design constraint which imposes tradeoff considerations. For example, in a photodetector circuit, where a resistor is typically utilized as a load, it is well known that to minimize the effect on the S/N ratio the impedance of the resistor must be large relative to that of the photodetector. Unfortunately, a large load resistor in a photodetector circuit operates to slow the response time of the circuit or reduce its bandwidth. Thus, it is an object of the present invention to provide a photodetector circuit with an improved S/N ratio and minimal degradation in response time.
In applicants U.S. Pat. No. 4,156,859, issued May 29, 1979 and 4,176,331, issued Nov. 27, 1979 there are disclosed classes of two-terminal active networks which simulate low-noise resistors. These circuits utilize a differential-input operational amplifier which senses the port input current and in response thereto generates a port output voltage. In this fashion a simulated resistor can be obtained. These simulated resistors are given the coined name "absorbor" with the "or" ending chosen to match the ending of other circuit components such as "resistor" and "inductor".